The present invention relates to a brake system of the ‘brake-by-wire’ type for actuating a motor vehicle brake system comprising a brake booster which is operable in response to the driver's wish both by means of a brake pedal and by means of an electronic regulating and control unit, with a means being provided to decouple a force-transmitting connection between the brake pedal and the brake booster in the ‘brake-by-wire’ operating mode, a master brake cylinder connected downstream of the brake booster in terms of effect, to the pressure chambers of which wheel brakes of the motor vehicle are connected, a pedal travel simulator which interacts with the brake pedal and due to which a resetting force acting on the brake pedal can be simulated in the ‘brake-by-wire’ operating mode independently of an actuation of the brake booster, and which can be enabled in the ‘brake-by-wire’ operating mode when the force-transmitting connection between the brake pedal and the brake booster is decoupled and can be disabled outside the ‘brake-by-wire’ operating mode, a first sensor to sense the brake pedal actuating travel, a second sensor to sense the travel of an output member (push rod) of the brake booster, and a third sensor (i.e. a pressure sensor) to sense the brake pressure prevailing in the system, the signals of which sensors are sent to the electronic regulating and control unit.
A brake system of this type is disclosed in the applicant's international patent application WO 2004/005095. It can be taken from the disclosure of the mentioned patent application that the electronic regulating and control unit initiates appropriate safety procedures, in particular a hydraulic fallback mode, on account of detected deviations between preset nominal values and actual values measured by means of the sensors. However, no hints can be taken from the document as to how failure of the hydraulic fallback mode being caused e.g. by the inclusion of air or the occurrence of leakages in the system, can be recognized by the driver or detected by the system, respectively.